THE EVIDENCE 63 



old, had a vascular zone only half a centimetre 

 (one-fifth of an inch) thick, about one-fifteenth 

 of the radius of the stem. Where the stem in- 

 creases much in diameter it often does so in an 

 unusual way, the original cambium ceasing to 

 be active, while a new cambium appears further 

 to the outside; this may take place several times. 

 Thus the wood comes to be built up of a num- 

 ber of successive layers, each produced by its 

 own cambium; this is the case in Cycas, Macro- 

 zamia, and other genera. There are many other 

 points of interest in the anatomical structure, into 

 which we will not enter now; we will go on at 

 once to the organs of reproduction. 



In the Cycads now living the sexes are always 

 separate, on different plants. Generally speak- 

 ing the fructifications are in the form of cones, 

 comparable to those we are familiar with in the 

 Coniferse, but simpler. This holds good for the 

 male fructification all through the family (figs. 1 

 and 3) ; in eight out of the nine genera the female 

 fructifications are also cones (fig. 4), but in Cycas 

 itself a different arrangement prevails (fig. 6). 

 The cones are stalked structures, borne at the 

 top of the stem; in fact, it appears that in many 

 cases, at any rate, the cone is really developed 

 from the apex of the whole plant, though 

 pushed aside by the subsequent growth of the 

 stem. 



